This invention relates to the art of electric heating systems. In preferred embodiments, the invention relates to an improved electric heating element for a hand-held, hot-melt glue gun.
Hand-held, hot-melt glue guns are provided with a chamber for heating and melting a solid, hot-melt material, which is typically in the form of a cylindrical stick. The material may be in any of several other shapes, including sticks having other cross sections, slugs, or chips. After melting, the composition is applied to a variety of surfaces.
The melting chamber is usually made of aluminum and has a cylindrical cavity for receiving the glue stick. The glue stick melts as it comes into thermal contact with the interior walls of the chamber, the heat being transferred primarily by conduction. A cartridge heater, a PTC (positive temperature coefficient) pellet, a rope heater or a resistance wire heater is typically provided to heat the chamber, and temperature control to prevent overheating is typically provided by a thermostat. One problem with the known temperature control, however, is that of overshoot, which arises from the fact that the chamber has thermal mass. Because of thermal inertia, the temperature of parts of the chamber, particularly those remote from the thermostat, will continue to increase even after the thermostat has terminated flow of electricity to the heater. The magnitude of the thermal inertia is a function of several factors, and the mass of the heating system is a primary factor.
Glue guns that are less expensive utilize PTC heaters because that type of heater is cost effective. One drawback is that PTC heating elements are of limited power, which can require the use of more than one element with consequent increase in cost. More power can be obtained by using a resistance heater with a discrete thermostat for temperature control, but the increased cost allows this in only more expensive systems. Further, the heat-up and melting performance of the known resistance heater systems is only marginally better than that of the PTC systems.
In prior systems, the chamber must be large enough to accommodate the heater cartridge and the thermostat. Thus, the prior systems have significant physical mass, resulting in significant thermal mass and thermal inertia. This mass results in a long warm-up time, for example, of 3-5 minutes, and increased overshooting. The problems created by this overshooting often outweigh the benefits obtained by the use of a PTC heater in the first place.
The usual heater is also provided with a silicone sleeve at the entrance to the heating chamber to provide a transition zone for the glue stick. The heating element must also heat the sleeve, by contact with the heating chamber, so that the glue stick will be able to xe2x80x9cbreakawayxe2x80x9d and enter the heating chamber. The required heating of the sleeve increases the warm-up time considerably.
Thus, there is a need for a heater that is capable of heating quickly, e.g., within 30 seconds, and effectively controlling temperature to significantly improve the convenience of glue guns.
In accordance with the invention, a heating chamber for a glue gun is capable of attaining an operating temperature in about 30 seconds. Further, the new heating chamber can be manufactured less expensively than conventional heaters and has a wider range of applications.
In a preferred embodiment of the invention, the heating chamber comprises a cylindrical tube that supports a coil of electrical heating wire. The cylindrical tube is preferably made of metal, such as aluminum, and provides a hollow core forming a cavity for receiving the glue to be melted. The tube may be of other materials, such as ceramic, high-temperature plastic, carbon-filled plastic, and the like. The core tapers slightly longitudinally (e.g., 2xc2x0) to provide draft and facilitate manufacture and may be die-cast or made by a screw machine. An important aspect of the inventive construction is that the thickness of the wall between the core and the exterior is preferably no greater than about 0.06 inches to reduce the mass of the heater markedly. The preferred chamber weighs approximately 17 grams, compared with about 45 grams for prior-art chambers of similar capabilities and having similar functions. One end of the chamber is designed to receive the glue stick, while the other is designed to receive a discharge nozzle. The nozzle may be an integral part of the chamber, but in the preferred embodiment the end of the chamber is internally or externally threaded for receiving a separate, threaded nozzle with a check valve to control dripping.
The outer surface of the tube is covered with a thermally conductive and electrically insulating film, such as a processed mica film. Other films may be used, such as a polyimide film sold under the trademark Kapton, a thin wall silicone film, a ceramic element or coating, a hardcoat anodized coating, and the like. As well, epoxy paint can be used as an electrical insulator. In the preferred embodiment, the tube is wrapped tightly with the film to ensure thermal contact, and the film is then secured to the tube by longitudinally spaced coils of 0.020 inch stainless steel lockwire. Then, a resistance heating wire, preferably made of 80% nickel and 20% chromium and sold under the trademark Nichrome, is wrapped over the electrically insulating film to form a tight coil extending longitudinally along the cylindrical tube. The individual coils are preferably spaced evenly along the major part of the tube, but it may be desired to vary the spacing to provide different rates of heating along the chamber. For example, it may be desired to provide more heat to the output end of the chamber. The size and length of the heating wire will vary depending on the specific application, including the heat to be generated and the physical dimensions of the tube.
While resistance wire is used in the preferred embodiment, it will be appreciated that other high power heating elements can be employed, such as a deposited carbon film, a woven resistance film, a Nichrome wire in a metal sheath, and the like.
The wire in the embodiment shown may be wound onto the tube in a variety of ways, such as by machine winding on a fixture resembling a lathe. Each end of the heating wire is secured to the tube, as by a lockwire also. Insulated lead wires are then attached to the ends of the heating wire, as by crimping. The coil of heating wire is then covered with a second layer of electrically insulating film, preferably the processed mica film, and secured as with lockwire in a manner similar to that of the first film.
Temperature control is provided by controlling power to the heating wire with a thermostat. This thermostat is physically attached to the insulated surface by a mechanical fastener, such as lockwire, and electrically connected in series with the heating coil to provide basic temperature control. By this arrangement, the thermostat picks up heat from the heating coil directly, which indicates the temperature of the chamber more quickly than do prior sensors that measure the temperature of the casting directly by being embedded in the casting. This construction reduces the time necessary to respond to the heat cycle to a few seconds.
Applicant has found that this arrangement greatly reduces overshoot problems and increases the ability of the thermostat to respond to the heating requirements. For example, cold glue presents a substantial thermal load that will absorb much of the heat provided by the coil. In this case, absorption of heat from the coil will keep it cooler, and the thermostat will heat more slowly. As the glue heats, it will absorb less heat from the coil, and the temperature of the coil will increase. Thus, as the glue heats up the thermostat will heat more quickly. In either instance, the rate at which the heating coil heats the thermostat is more closely tied to the rate at which it heats the load than in systems where the thermostat is located in the metal casting.
A basic concept of the invention is to provide a low mass, high power, fast-heating chamber controlled by a thermostat that measures the temperature of the load indirectly by reacting to the temperature of the heating element without significant overshoot. The thermal characteristics of the thermostat are designed such that the temperature difference between the heating coil and the melted glue is the same, or essentially the same, as the temperature difference between the heating coil and the thermostatic element held in the thermostat""s casing, when the glue is at the desired temperature. Thus, the mass of the thermostatic element and its casing, and the casing""s area of contact with the coil should be such that the thermostat reacts to the heating coil in a manner similar to that of the cylinder and the heated glue. This is particularly useful in the environment of the present invention because the heating coil is of such high power compared to the mass of the cylindrical tube that any lag in detecting the temperature of the tube can result in melting the tube.
Other types of temperature control can be used. For example, a thermistor placed in a metal casing and designed to absorb heat as set forth above could be used. In this example, a negative temperature coefficient (NTC) thermistor, whose resistance decreases with increasing temperature, senses the temperature of the system, and a solid state comparator circuit switches a relay or Triac on or off to control the heating element.
Applicant has determined that a preferred circuit for providing power to the resistance-heating element includes a xc2xd wave rectifier to reduce the voltage and resultant power delivered to the heater. This is preferred because it has been found more difficult to size the heating wire for glue guns that operate on 120 VAC or higher voltage. Use of a rectifier circuit, allows the heater wire to be of heavier gauge and, therefore, more durable and easy to handle, even in small wattage ranges. One reason the heavier wire is more durable is that it is subjected to less thermal stress.
In a preferred embodiment, 33 Ohms of #30 Gauge wire (about 6.4 Ohms per foot) was used to deliver about 190 Watts of power to a cylindrical tube weighing 17 grams. This arrangement provided 10-11 Watts per gram. A conventional arrangement provides about 40 Watts for a heater weighing about 45 grams, or a ratio of about 1 watt per gram. Thus, a system in accordance with the invention heats about ten times faster than the conventional system.
The preferred embodiment described has been found to heat to operating temperature consistently in less than 30 seconds and to provide larger continuous melting than conventional systems. Thus, warm up time has been reduced from 3 minutes to 30 seconds. Further, the melt rate was not compromised because the system is more responsive to temperature drops during normal usage.